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WASHINGTON — A "specific, credible" threat has emerged surrounding the 10th anniversary of the Sept. 11, 2001, attacks, U.S. officials said Thursday.

But they emphasized that there was a great deal of uncertainty about the threat and said they were acknowledging it out of an abundance of caution. NBC News said police in New York City and Washington had been alerted.

New York Mayor Michael Bloomberg said late Thursday 9/11 commemorations would go on as planned under heightened police presence already boosted for the weekend.

Recent intelligence obtained within the past day, and originating from the tribal areas of Pakistan, advised the United States of a plan to set off car or truck bombs in Washington or New York around the time of the 9/11 anniversary, officials told NBC News.

The information indicated that three people would travel to the U.S. from Pakistan to carry out an attack, they said. However, the information included very little else in the way of specifics — in terms of timing or target.

Law enforcement officials were being alerted to be on the lookout for three men "of Middle Eastern descent" and potentially traveling in a van, they said.

While the information had some specificity and the source has been credible in the past, it's uncorroborated, officials warned.

However, given that documents found when Osama bin Laden was killed indicated his desire for an attack on the 9/11 anniversary, and further given concerns about possible retaliation for bin Laden's death, officials decided to pass this information along to local law enforcement agencies.

"Now is not the time not to tell them about it," one official said Thursday night.

Police in Washington and New York were told earlier Thursday to expect an alert, and some officers said they were told to stay on duty longer.

"In this instance, it's accurate that there is specific, credible but unconfirmed threat information," the Department of Homeland Security said Thursday in a prepared statement. "As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days. Sometimes this reporting is credible and warrants intense focus, other times it lacks credibility and is highly unlikely to be reflective of real plots under way.

"Regardless, we take all threat reporting seriously, and we have taken, and will continue to take all steps necessary to mitigate any threats that arise. We continue to ask the American people to remain vigilant as we head into the weekend."

An administration official told NBC News the threat "so far" was a 5 to 6 on a scale of 10.

President Barack Obama was briefed on the specific threat information Thursday morning and was updated on it throughout the day, a White House official said. The U.S. government "already significantly enhanced its security posture in advance of the 9/11 anniversary to protect the country against possible terrorist threats. Nevertheless, the president directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information."

One official told NBC News that discussions are under way about whether to raise the threat warning status, but no decision has been made.

Earlier Thursday, Homeland Security Secretary Janet Napolitano said "in the intel world there is lots of chatter and we are taking it very seriously."

The threat was revealed as officials said they already were adding thousands of extra police, cameras and weapons around New York, Washington, and western Pennsylvania for Sept. 11 anniversary attacks.

Taking no chances in NY
The New York Police Department, as a result of the threat information, is stepping up its protective measures, some visible to the public and some not, starting now, New York Mayor Michael Bloomberg and Police Commissioner Raymond Kelly told a Thursday night news conference.

They also urged people to go about their business but remain vigilant.

"I will take the subway tomorrow and feel just as safe as I did this morning," Bloomberg said.

"If you see something, say something," he said, noting the possibility of people calling in reports unattended packages on subway platforms or cars parked illegally.

Kelly said police would be held for an extra 4 hours of duty beginning Friday, effectively increasing police patrols by a third. He also said the public will be subject to vehicle checkpoints around the city; more bags will be inspected on the subway system; more bomb-sniffing dogs will be on patrol; more bomb sweeps will be conducted in parking garages; more illegally parked cars will be towed, and more radiation detectors would deployed with officers.

Special attention will be paid to bridges, tunnels and infrastructure, they said.

"There's absolutely no need for any panic," Rep. Peter King, R-N.Y., chairman of the House Homeland Security Committee, told NBC News earlier. He said he was briefed on the threat.

Kelly earlier outlined security procedures earlier this week at a Manhattan Institute conference on terrorism, the New York Daily News reported.

Kelly told the group police are not taking any chances for a Sunday observance at the World Trade Center that Obama and former President George W. Bush plan to attend.

Mark Lennihan
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AP

A New York City police officer stops Xiang Mao, left, and Lexi Li on Thursday along the perimeter of the World Trade Center, after the officer's hand-held radiation detector indicated that one of the two may have had radioactive material. Xiang Mao explained that he had been injected with radioactive material for medical tests Wednesday. The two tourists from Canton, China were allowed to continue on their way.

Kelly said Ground Zero would be surrounded by a frozen zone. Thousands of cops will be on duty, some carrying radiation detectors and sniper rifles. Hundreds of surveillance cameras will monitor the site.

Kelly said quick-reaction forces will be assigned outside lower Manhattan.

Cops trained to use heavy weapons after the Mumbai 2008 terror attacks will supplement Emergency Services officers, he said.

"Bomb squad officers and hostage negotiators will also be attached to these quick reaction groups, as will highway patrol officers to ensure these teams can get to any point in the city as quickly as possible," Kelly said.

A police source told the Daily News that two teams of these officers will be in place by the United Nations and near one of the bridges leading to an outer borough.

Police scuba divers will also examine the USS New York, made of World Trade Center steel, at its West 48th Street dock, Kelly said.

Capital concerns
Some District of Columbia police officers received an internal memo from department management on Thursday saying that the city additional staffing would be required due to the "credible threats," according to a law enforcement source who had seen the memo.

District Police Chief Cathy Lanier earlier told The Associated Press that every one of her 3,800 officers would work at some point during remembrances.

"You'll see mass transit, you'll see restaurants, hotels, sporting events — any place where there's a crowd, we're going to have an increased presence," Lanier said.

People walking or traveling near the U.S. Capitol will see more uniformed and plainclothes officers, additional police cruisers and may notice a bomb squad or SWAT team in areas of Capitol Hill where they wouldn't ordinarily be seen, said Sgt. Kimberly Schneider, a U.S. Capitol Police spokeswoman. The U.S. Park Police, which has jurisdiction over D.C. landmarks including the National Mall and the Washington Monument, is also on alert.

"We're well aware that this anniversary does bring with it a lot of emotion and concerns, but we're certainly addressing it," said Park Police spokesman David Schlosser.

National Park Service’s Flight 93 National Memorial in Stonycreek Township near Shanksville will be unveiled Saturday during events that are expected to draw 10,000 people, including Vice President Joe Biden. Obama is expected at events on Sunday.

Cory Angell, a PEMA spokesman, told radio station WHYY the center nearest to the site of the Flight 93 memorial in Somerset County will be activated at a Level 2 — meaning key emergency staffers from different state departments will be on duty.

"In the western part of Pittsburgh, of course — Flight 93 and the event that they're having in memory of those lost on 9/11 — being a significant event, that EOC will be activated at a Level 2, as well," he said.

The Flight 93 memorial's website notes that security checkpoints will be located at ceremony entrances, chairs will be permitted but not in bags; coolers are banned.

8:30 a.m.-5:30 p.m. - Bill Clinton will speak at this week's dedication of the Flight 93 National Memorial in western Pennsylvania. The former president will join former President George W. Bush, Vice President Joe Biden, Speaker of the House John Boehner and Flight 93 family members at Saturday's event (which runs 10 a.m.-5:30 p.m.). Sunday is the 10th anniversary commemoration of the Sept. 11 terrorist attacks. Events will take place 8:30-11 a.m. and 1:50-5 p.m. The events are open to the public, and security is expected to be tight.

8:30 a.m.-1 p.m.: The National September 11 Memorial in New York will be dedicated during a ceremony for victims' families. President Barack Obama and former President George W. Bush plan to attend. The memorial opens to the public the following day.

9:30 a.m.: The Pentagon Memorial will have an invite-only ceremony for families of 9/11 victims at 9:30 am. The Pentagon Memorial will re-open to the general public following the ceremony.

1-4 p.m.: HandsOn Greater DC Cares and Serve DC - the Mayor's Office of Volunteerism will commemorate the 10th anniversary of 9/11 by hosting Tribute to Service – Honoring the Victims, Survivors, and Heroes of 9/11 at Freedom Plaza in downtown Washington, D.C. from 1-4 p.m.